The present invention relates to hydrostatic transmission systems and controls, and more particularly, to closed loop hydrostatic transmissions of the type including a charge pump which serves as the sole source of make-up fluid to the closed loop, and as the sole source of control fluid to the system controls.
A typical closed loop hydrostatic transmission system of the type to which the present invention relates is illustrated and described in U.S. Pat. No. 3,359,727. In such systems, the closed loop and the various system components are protected from excessive pressures by means of a pair of high-pressure (over-pressure) relief valves which are typically included in a valve block associated with the system motor. Such relief valves are sized to be able to communicate full system flow from the high-pressure side of the loop to the low-pressure side, and as a result, are quite large and expensive.
Many such closed loop hydrostatic transmission systems also require some sort of power-limiting control, i.e., a control system which will automatically destroke the variable pump whenever the total input power consumed by the pump exceeds a predetermined maximum. Such power limiting controls tend to be fairly complex and expensive, because such systems require, as inputs, factors such as system pressure, pump displacement, and maximum predetermined power setting. See for example U.S. Pat. No. 3,884,039, illustrating such a control system including both hydraulic and mechanical controls.